1. Field of the Invention
The present invention generally relates to devices used to mark and locate obscured objects, and more particularly to an electronic marker device which is attached to a cable closure or other buried conduit.
2. Description of the Related Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, optical fiber, power and television. It often becomes necessary to locate defective or damaged cables, pipes, etc., in order to repair or replace them. Conversely, it is important to know with as much accuracy as possible the approximate vicinity of such items in order to avoid disturbing them when digging or excavating for other purposes. Above-ground marking devices may be installed immediately after the conduit is buried, but they are often lost, stolen, or destroyed after a short period of use. Therefore, it is common to use underground marking devices or systems to enable the later location of a section or feature of an underground utility.
In the past, three different approaches have been used to indicate the presence of buried conduits, namely, warning tapes, trace wires, and electronic marker systems. A warning tape is simply a band of plastic which is placed above the conduit before burial. These tapes are used to alert the excavation team of the presence of the conduit before any damage thereto might occur. As the backhoe or other mechanical digger excavates the site, it will hopefully uproot a portion of the warning tape prior to contact with the conduit. The primary disadvantage of (non-metallic) warning tapes is that they cannot be detected by any surface instrumentation.
A single trace wire is sometimes buried with a utility line. The trace wire is used as a conductor for an AC signal which is applied to the wire at one accessible end, and then acts as an antenna and radiates an electromagnetic field above ground along its entire length. The electromagnetic field may be detected with an appropriate receiver, and the underground path of the line thereby determined. The earliest cable locators used a single sensor which detects a single null or peak (depending upon the orientation of the sensor) as the unit passes near the cable. Many later devices use two or more sensors that combine the signals to provide an indication of conductor proximity. The most common sensors are ferrite-core antennas, i.e., inductors. Although the conduit itself may act as a conductor (i.e., when steel pipe or copper wire cabling is used), most conduits are non-conductive and therefore require a trace wire. There are three significant disadvantages in the use of a trace wire. First of all, it is necessary to provide above ground access to the trace wire in order to couple the AC signal thereto. Secondly, if a break occurs in the wire (due to excavation, or natural causes such as corrosion, earth movement or burrowing animals), then the wire becomes useless. Finally, the trace wire is too thin to imprint a warning message thereon, precluding any visual warning. Additionally, a receiver cannot distinguish the trace wire from any other conductor in the vicinity.
Electronic marker systems for locating buried objects are known in the art, and generally consist of two types, namely, active and passive markers (transponders). Active markers require the use of a power supply which amplifies a signal source (usually an AC signal). The signal is radiated by the underground marker and detected by a receiver unit above ground. Passive markers, in contrast, have no power supply, but rather operate in a resonant mode, responsive to a transmitted electromagnetic field.
A passive marker is basically a wire coil and capacitor surrounded in a protective envelope, which is then buried adjacent to the cable, pipe, or other object to be located. The marker is self-contained, with no external, accessible connections. Passive markers are activated by radiating a signal into the ground in the area where the marker is expected to be found. The signal is emitted via an inductive coil held close to the surface (the transmitter portion of a transceiver). When the coil is directly over, or near, the passive marker (which is itself an inductive coil), the marker accepts energy within its bandpass and stores it, reaching a sustained amplitude during the transmission cycle. When the transmission cycle ends, the marker re-emits the energy at the marker's resonant frequency with an exponentially decaying amplitude. A second coil within the transceiver unit acts as a receiving antenna which detects the re-radiated energy, alerting the locating technician with an audible tone or other indicator means.
There are several kinds of passive transponders for different applications. These include small, near-surface markers for locating objects just inches below the surface, medium size or mid-range markers, full-range markers for locating more deeply buried objects, and self-aligning markers such as the so-called ball marker which supports the marker coil horizontally, regardless of the orientation of the housing. There are hybrid systems wherein, for example, a signal is applied to a buried conductor (cable or trace wire), and coupled through the conductor to one or more markers buried adjacent the conductor. Also, a marker can be used to couple one conductor to another, so that the test signal may be conveyed to the second conductor without a direct physical connection. All of these markers generally float around the underground feature in the soil, and are subject to soil movement.
Electronic markers, as well as warning tapes, are usually color-coded according to the particular type of utility line they mark. Specifically, gas line markers are yellow; telephone cable markers are orange; waste water tunnel markers are green; water pipe markers are blue; and power supply markers are red. Similarly, the passive marker is electronically coded by tuning the coil for a specific resonant frequency. Five distinct frequencies have been designated: 83.0 kHz for gas; 101.4 kHz for telephone; 121.6 kHz for sewage; 145.7 kHz for water; and 169.8 kHz for power. In this manner, a locating technician searching for, say, a gas line, cannot accidentally activate a telephone marker since his transmitter will only be sending out an 83 kHz signal, which is not within the bandwidth for a telephone marker tuned for 101.4 kHz. These frequencies have been designated by convention, and are not meant to be restrictive.
While passive electronic markers have several advantages over warning tapes and tracing wires, they are still subject to certain limitations relating to, for example, the desired resonant frequency of a particular marker. In a passive marker, which is essentially an LC circuit, the resonant frequency f is given by the equation f=1/(2π√LC) where L is the inductance of the wire coil and C is the capacitance of the capacitor. This frequency must be closely controlled in order to adhere to the foregoing tuning conventions and to provide a return signal of maximum amplitude; however, the actual frequency is affected by component construction, manufacturing tolerances, operating temperature, aging, placement and other factors.
One problem associated with using passive or active transponders for remote identification of buried utility structures is that, if the structure is itself metallic (electrically conductive), then it will influence the detection process by causing a variance in the magnetic lines of flux from metallic sources. It is generally assumed that, to provide accurate locatability, a transponder must be separated from other metallic structures by a minimum of about four inches to avoid magnetic or electromagnetic interference. This effect can in particular be a problem in marking service drops or splices for a telecommunications cable wherein the cabling has conductive elements or sheaths. Current methods for locating service drops are highly dependent on field crews for correct placement of transponders for accurate locating (maintaining a predetermined separation distance from metallic components). Even with correct placement procedures, movement can occur in a filled ditch or hole as a result of loose unpacked soil, freeze/thaw cycling, water erosion, and other causes.
An invention which addresses this problem is described in U.S. Pat. No. 6,271,667. One device disclosed therein combines an electronic marker with a shield or cover that protects a cable closure. The marker is located in a raised portion of the closure guard to reduce interference with metallics in the closure, but there are still problems in using that device. The indicated height of the raised marker is only 3″ which may be insufficient to prevent interference. The closure guard is also attached by tie wraps which must be tightly secured or else the device has too much freedom and can move to one side of the cable closure during or after burial.
In general it would be preferable to minimize the involvement of field personnel in deploying markers. Unfortunately, it is difficult to pre-install markers for a number of reasons. Many closures or other utility structures are fabricated in situ, i.e., they are not factory made. There are, however, cables that have multiple, factory-installed closures for drop points or splices used in wiring a subdivision. These preconnectorized cables are wound on large (36″ diameter) reels. Markers could be integrated into such factory-made closures but if they were positioned too close to a closure this would result in interference, and if they were positioned distant from the closure the result would be too bulky to be efficiently wound on a reel.
In light of the foregoing, it would be desirable to devise an improved utility marker that could accurately locate a transponder without dependency on craft skill at a proper distance from a buried component of a utility infrastructure, and in particular raised vertically above the infrastructure to facilitate relocation. It would be further advantageous if the utility marker could be pre-attached to a utility cable or conduit without adding significant bulk.